2006 Mercedes-Benz SL and SLK diesel

MUNICH At this year’s Geneva show, Mercedes reaffirmed its commitment to diesel engines by unveiling the two best production-ready, diesel-powered cars ever made, the SL400 CDI Biturbo and the SLK320 CDI Triturbo. Although Mercedes called both roadsters “studies,” the SLK version is expected to go on sale in Europe sometime in 2006. The SL400 is unlikely to reach dealers, since Mercedes feels that diesels are not yet appropriate for the SL’s brand image, but spending time with the cars proved to us that Mercedes is serious about moving the diesel away from its fuel-miser image and toward the realm of power, torque, and driving pleasure.

We pop the hood of the SLK, take off the plastic cover, and inspect the ultimate in artificial aspiration: a small turbo on the left, a small turbo on the right, and a big turbo right in the middle of the 282-hp, 72-degree V-6. The SL diesel, on the other hand, makes do with only two turbos for its 4.0-liter V-8.

Push the SL400’s shift lever into first gear, switch off ESP, hold the brakes with your left foot, and lay on about 1000 revs. When the traffic signal turns green, the 311-hp SL400 peels rubber with an even greater vengeance than the 500-hp, supercharged, 5.5-liter SL55 AMG. With the help of 539 lb-ft of torque, the diesel-engined SL storms from 0 to 60 mph in 5.8 seconds, putting it within about a second of the V-12 SL600 and the SL55 AMG. The SLK Triturbo does even better than that, eclipsing its big brother by an incredible 0.5 second. Both roadsters return sensational combined fuel economy of 37 mpg.

However, on-off idle clatter, rough full-load combustion, and occasionally brutal accelerator tip-in and tip-out prove that diesels still are not perfect. Yet these engines perform remarkably well, even under the hoods of two high-performance sports cars.

There are no plans to bring either diesel roadster here, but we will get Mercedes’ new, 3.0-liter, single-turbo V-6 diesel-producing 221 hp and 376 lb-ft-in the ML320 CDI next summer and the E320 CDI next fall, with an R320 CDI possibly to follow.

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